1. Field of the Invention
The present invention relates to monitoring respirations of a patient, and more specifically to impedance pneumography, which is a technique for monitoring respirations by monitoring changes in electrical impedance caused by the expansion and contraction of the thoracic cavity of the patient during respiration.
2. Description of the Prior Art
Typically, a differential, constant amplitude AC examination current is applied to the body of a patient for detecting patient respirations by measuring changes in the transthoracic impedance of the patient. The examination current is applied to the patient by two of the electrodes normally used for ECG monitoring. The examination current is passed through the thoracic cavity of the patient and, due to the constant amplitude examination current passing through a transthoracic impedance that changes with patient respirations, voltage modulations are created at the ECG electrodes in accordance with the patient respirations. The resulting voltage modulations are typically detected by a synchronous voltage detector, connected to the same pair of electrodes as was used for applying the examination current, such as the right arm (RA) and left arm (LA) electrodes.
The cables used for connecting the examination current and voltage detector to the patient contain capacitive reactances which tends to shunt a portion of the examination current around the patient. One effect of this shunting is that the system gain, as expressed in Volts/Ohm, will have a dependency on the baseline level of the transthoracic impedance, as well as the level of any impedance placed in series with the patient, such as resistances for protecting the monitoring circuitry from defibrillator voltages which may be applied to the patient. This dependency makes detecting the small respiration induced changes in impedance more difficult (that is, such a dependency making it difficult to set absolute signal detection threshold levels). Another effect is that the induced voltage becomes sensitive to changes in the frequency of the examination current. Any phase or frequency jitter in the examination current signals, or the clock signals used to detect the examination current, will be converted to a voltage noise during detection by the synchronous voltage detector. The above-noted shunting of the examination current, changes in the system gain, and voltage noise result in signal artifacts in the induced voltage modulations which reduce the accuracy of the respiration detection circuitry.
It is an object of the present invention to provide an AC examination current source which will solve these undesirable effects, while providing a current source which is relatively low in cost, which will present minimum load to ECG signals acquired by the electrodes, and able to be easily manufactured using integrated circuit technology.
A prior art respiration monitor manufactured by Hewlett-Packard (believed to be sold under the trademark CLOVER) applies a fixed frequency sine wave to an impedance bridge, one leg of which is connected to the thoracic cavity of a patient via a transformer. It is believed that the magnetizing inductance of the transformer may be intended to at least partially compensate for the capacitance in the patient cable although this is not specifically known. The output of the bridge is fed to a synchronous detector for developing the respiration signal in accordance with known techniques. Although the transformer in this monitor may provide some compensation for the capacitance of the patient cable, the technique undesirably requires the use of a transformer, which is bulky and not well suited for incorporation with integrated circuit technology. Additionally, this technique requires a sinewave examination current, which is somewhat difficult and costly to generate using digital circuitry.